Does the trans always work fine in 1st and 2nd? Even right after start-up?
Is the throttle pressure (aka kickdown) cable installed and properly adjusted? If the throttle pressure cable is not hooked up or inoperative, the front clutch will be smoked and the trans needs to come out for repair.
Does the trans slip in Reverse too?
I recommend getting a 300psi gauge and a 8' length of hose good for at least 250 psi. You will also need fittings to connect the hose to the 1/8" NPT female threads on the trans.
If the trans operates fine every time in 1st and 2nd, hook the gauge hose onto the line pressure port, bring the gauge into the cab, and go for a test drive.
With a steady throttle position, watch the pressure gauge during:
1st gear accelerating
the shift from 1st to 2nd
2nd gear accelerating
the shift from 2nd to 3rd
while in 3rd and slipping
Don't slip the trans any more than necessary, you will make the repair more expensive.
If the throttle pressure cable is OK, my gut feeling is that they damaged the outside lip seal on the front clutch drum at the rebuild. Sometimes that seal is tough to get in, because it is inaccessible during installation. Forcing the piston in will damage the seal.
The front clutch is used for 3rd gear as well as OD and reverse. Trans needs to come out to service the front clutch.
I have used the 1/4" black plastic air brake tubing from big trucks in a pinch.
